1. Field of the Invention
The present invention relates to an optical disk recording/reproducing apparatus for recording and/or reproducing an information signal onto/from an optical information recording medium such as an optical disk and more particularly to an optical disk recording/reproducing apparatus with a tilt sensor for detecting a tilt between the optical information recording medium and an objective lens.
2. Description of the Related Art
In recent years, the demand for high density recording to handle a high precision still picture, a video and the like, by an optical information recording medium such as an optical disk and digital versatile disk (DVD) has increased in the information recording and reproducing field. To enable such high precision recording, an optical disk recording/reproducing apparatus capable of realizing a large capacity recording is desired.
Examples of methods to achieve this requirement, include selecting a laser source with a shorter wavelength equipped with an optical pickup unit, and reducing a diameter of a beam spot by increasing the numerical aperture (NA) of the objective lens.
However, if the beam spot diameter is reduced or when the disk track pitch is increased in density, the interference of information signal between a track presently accessed and the adjacent tracks increases, that is, the "crosstalk", or an interference between adjacent signals in the same track increases. These interferences are generated due to a relative tilt between an optical axis of the objective lens of the optical pickup unit and the optical information recording medium (hereinafter referred to as "tilt"). As a result, accurate recording and reproducing of an information signal with respect to the optical information recording medium can not be obtained.
The relationship between an allowance value .theta. relative to the tilt, a laser wavelength .lambda. and a numerical aperture NA is expressed as follows: EQU .theta..varies..lambda./(NA).sup.3 (Equation 1)
Therefore, it is necessary to achieve an accurate tilt servo for a high density recording of the disk. Also, the optical disk recording/reproducing apparatus must be provided with a tilt detector capable of detecting an accurate tilt amount while recording and reproducing the information.